Before a network of surface irrigation system is designed, the parameters taken into consideration in designing the irrigation system of the surface is determined. The parameters are the data about the condition of the area to be built irrigation system and the surrounding environment. These data should be investigated accurately in accordance with the conditions of the field. The data required in planning irgasi system is as follows, namely the data (1) hydrology, (2) topography and (3) geological engineering.
The parameters of hydrological data which is essential for planning irrigation networks are rainfall, evapotranspiration rate, the peak discharge and daily discharge, sediment transport.
a. Rainfall
Analysis of precipitation is done with a view to determining:
Effective precipitation to calculate irrigation needs. Effective rainfall or mainstay is part of the overall rainfall is effectively available for crop water requirement.
The rainfall excess (excess rainfall) is used to calculate the need for the disposal or drainage and discharge (flood).
For the analysis of effective rainfall, rainfall in the dry and rainy season will be very important. For more precipitation, rainfall in the rainy season is the months of rain. For both these goals daily rainfall data will be analyzed to obtain an acceptable level of accuracy. Daily rainfall data covering a period of at least 10 years will be required.
b. Evaportanspirasi
Analysis of the evapotraspirasi needed to determine the magnitude of the rate of evapotranspiration of plants that will be used to calculate irrigation needs, and if it is necessary to study the water balance in the watershed. These studies may be done when the data stream is not available in sufficient quantities.
Climatic data required for this calculation is that relating to:
The temperature, which is the maximum daily temperature, minimum and average.
relative humidity.
The sun is ever the sun shines during the day.
wind conditions, including wind speed and direction.
daily rate of evaporation is evaporation.
The data mentioned above is the standard for stations agrometerologi. Recording time period for analysis sufficiently precise and reliable is about ten years.
c. Flood Plans
The maximum flood discharge plan is in the creek or natural channel with an average return period predetermined that can be streamed without endangering the stability of irrigation networks and buildings. Flood discharge determined by analyzing the peak discharge, and is usually calculated based on the daily observation of the water level. For the purposes of analysis sufficiently precise and reliable, the data records used must cover at least 20 years.
Another factor that is more difficult is the absence of observations of water level or the peak discharge of the record available. Peak discharge data that only covers a short period would make it difficult and even dangerous for the observer. The prices of the discharge plan is often determined using empirical hydrological methods, or analysis by linking prices to the price of rainfall flooding.
d. Debit Mainstay
Debit mainstay (dependable flow) is the minimum flow for the river could have taken fulfilled predetermined that could be used for irrigation. The possibility of unmet set to 80% (likelihood that the river flow is lower than the discharge mainstay is 20%). Debit mainstay determined for the middle period – monthly. A minimum flow of the river is analyzed on the basis of the daily discharge data stream. In order to sufficiently precise and reliable analysis, record the necessary data must cover a period of at least 20 years.
If these requirements can not be met, the analytical and empirical hydrological methods can be used. In calculating discharge mainstay, should be considered the necessary water from the river downstream retrieval. In practice it turns out debit mainstay from time to time decrease with decreased function of the catchment area. Decrease mainstay discharge can cause the performance of irrigation is reduced resulting in a reduction in paddy fields. Anticipation of this situation needs to be done by entering a correction factor the amount of 80% – 90% for debit mainstay. The correction factor depends on the condition of the watershed changes.